1. Field of the Invention
The present invention relates to a power source circuit, an electronic device being equipped with the power source circuit, and a method for controlling the power source circuit, and more particularly to the power source circuit being suitably used when supply power is fed to a load whose load current is intermittently changed such as a power source section of a power amplifier, for transmission of a radio wave, being embedded in a portable cellular phone, to the electronic device having the above power source circuit and to the method for controlling the power source circuit.
The present application claims priority of Japanese Patent Application No. 2001-284088 filed on Sep. 18, 2001, which is hereby incorporated by reference.
2. Description of the Related Art
In a power source circuit using a chemical battery cell, when a load whose load current is intermittently changed is connected thereto, since an internal impedance of the chemical battery cell is comparatively large, a phenomenon occurs in which a voltage drops instantaneously at a same time when an instantaneous increase occurs in a load current. To solve this problem, conventionally, a capacitor having a comparatively low impedance is connected, in parallel to, the chemical battery cell. As a result, even if a load in which a synthetic impedance being lower than an internal impedance of the chemical battery cell is formed is connected to the chemical battery cell, an instantaneous drop rate is lowered when compared with a case in which the same chemical battery cell is singly used.
The conventional power source circuit of this kind, as shown in FIG. 7, is provided with a chemical battery cell 1 and a capacitor 2 being connected in parallel to the chemical battery cell 1 and a load L is connected to both the chemical battery cell 1 and the capacitor 2. The chemical battery cell 1 is made up of a secondary battery cell such as a nickel cadmium cell, nickel hydrogen cell, lithium ion cell, or a like, or an alkaline primary battery cell and saves a specified amount of energy and generates an electromotive force (that is, a voltage) V1 to feed it to the load L. The chemical battery cell 1 has an internal impedance 1a. The capacitor 2 is made up of, for example, an electric double-layer capacitor and is charged at a voltage V1 of the chemical battery cell 1 to accumulate power and feeds the accumulated power to the load L. The capacitor 2 has an internal impedance 2a. The load L is, for example, a power amplifier, for emitting a transmission radio wave, embedded in a portable cellular phone in which a load current is changed intermittently and a pulse-like load current IL flows therein.
FIG. 8 is a timing chart of signals explaining operations of the conventional power source circuit of FIG. 7. A current and a voltage are plotted as the ordinate and time is plotted as the abscissa. Operations of the power source circuit shown in FIG. 7 are described by referring to FIG. 8. At a time t1, a load current IL increases instantaneously and the voltage V1 of the chemical battery cell 1 drops from a voltage value Va to a voltage value Vb. In this case, an internal impedance Z of the power source circuit is given by a following equation:Z=R1+R2/(R1+R2)  Equation (1)where “R1” is a value of the internal impedance 1a and “R2” is a value of the internal impedance 2a. Thus, the internal impedance Z is smaller than the internal impedance 1a. A drop rate of a voltage value Vb is smaller compared with that of a voltage value Vc to be produced when the power source is made up of the chemical battery cell 1. At a time t2, the pulse-like load current IL decreases instantaneously and voltage V1 of the chemical battery cell 1 returns from the voltage value Vb to the voltage value Va. A voltage V2 of the capacitor 2 changes in the same manner as above.
Thus, when the capacitor 2 is connected in parallel to the chemical battery cell 1, since a drop rate of the voltage value Vb is small when compared with the voltage value Vc, if the chemical battery cell 1 is made up of the secondary battery cell, time being usable for every single charge cycle becomes longer when compared with a case in which the power source circuit is made up of only the chemical battery cell 1. Moreover, when the chemical battery cell 1 is made up of the alkaline primary battery cell having a comparatively high internal impedance, a life of the power source circuit becomes longer when compared with the case in which it is made up of only the chemical battery cell 1.
However, the conventional power source presents the following problems. That is, in an electronic device having the power source circuit shown in FIG. 7, whether a remaining capacity of the chemical battery cell 1 is sufficient or insufficient is judged based merely on a drop in the voltage V1 and, therefore, there occurs a case in which, even if an instantaneous drop of the voltage V1 occurs, the remaining capacity of the chemical battery cell 1 is judged to be insufficient. However, if the chemical battery cell 1 is made up of, for example, the alkaline primary battery cell, even if a remaining capacity of the chemical battery cell 1 is judged to be insufficient to operate a certain electric device, the same remaining capacity thereof may be judged to be sufficient to operate another certain electric device. This phenomenon indicates that capacitance of the chemical battery cell 1 has not yet been completely exhausted. That is, the chemical battery cell 1 is judged to be at an end of its life in a state in which discharging depth (that is, a ration of already-discharged capacity to a rated capacitance) of the chemical battery cell 1 is small, which, as a result, presents a problem in that an efficiency of using energy of the chemical battery cell 1 is lowered. Moreover, when the chemical battery cell 1 is made up of, for example, the secondary battery cell, nickel cadmium cell, nickel hydrogen cell, lithium ion cell, or the like, if the chemical battery cell 1 has not run out of its capacitance, time being usable for every singly charge cycle become extremely shorter than time usable in an original cell capacitance. Furthermore, there is a problem in that voltage of the capacitor 2 may be unable to respond to an intermittent change in the load current IL, depending on a capacitance of capacitor 2.